1. Field of the Invention
The present invention generally relates to optical disks, clock generation methods, signal generation methods, and optical disk devices, and more particularly, to an optical disk recording thereon multilevel information, a clock signal generation method of generating a clock signal that is used when reproducing the information from the optical disk, and an optical disk device reproducing the information from the optical disk.
Additionally, the present invention relates to an optical disk recording thereon information that is multileveled to three or more values, a signal generation method of generating a signal used when accessing the optical disk, and an optical disk device that accesses the optical disk.
2. Description of the Related Art
Recently, with the progress of digital technologies and the advancement of data compression technologies, optical disks such as a CD-ROM (CD) and a DVD-ROM (DVD: digital versatile disk), which has the same diameter as a CD-ROM and can record about seven times as much data as a CD-ROM, have been drawing attentions as media for recording information (hereinafter also referred to as “content(s)”) such as music, movies, photographs and computer software. As the prices of such optical disks fall, optical disk devices have been widely used that reproduce contents recorded on optical disks.
A spiral or concentric pit array is formed on a recording surface of a read-only optical disk such as a CD-ROM and a DVD-ROM. Information is recorded by the lengths of pits, the lengths between pits, and combinations of these. In this case, the information is converted into combinations of two kinds of numeric values (binary values), 0 and 1, and is written on an optical disk. Hereinafter, such a recording method is referred to as a binary recording method.
Generally, meandering (wobbling) track is formed on a recording surface of a recordable optical disk such as a CD-R (CD-recordable), a DVD-R (DVD-recordable) and a DVD+R (DVD+recordable) and a rewritable optical disk such as a DVD-RW (DVD-rewritable) and a DVD+RW (DVD+rewritable). The wobble is partially modulated by a predetermined method, and information is added to the modulated portion. For example, in a DVD+R and a DVD+RW (hereinafter also referred to as “the DVD+ system” for convenience), address information is added to such modulated portion by the phase modulation method.
Thus, in an optical disk device corresponding to the DVD+ system, a so-called wobble signal corresponding to the wobble shapes of tracks is detected from the light emitted from a light source and reflected by the tracks, a clock signal and the like are generated from the wobble signal, and phase demodulation is performed on the wobble signal in synchronization with the clock signal, thereby obtaining the address information. Based on the address information and the clock signal etc., the reproduction timing and the recording timing are controlled.
In an optical disk, information is recorded by lengths of mark regions and space regions, each having a different reflectivity from each other, and combinations of them. In this case, information is written on the optical disk by being converted (digitized) into combinations of two kinds of numeric values (binary), 0 and 1. Hereinafter, such a recording method is referred to as the binary recording method.
The amount of information (contents) has been increasing year by year, and it is expected that an optical disk can record a further amount of information. In order to increase the amount of information that can be recorded on an optical disk, it is conceivable to convert information into combinations of three or more kinds of numeric values and write it on an optical disk. Thus, various related techniques have been intensively developed so as to put them into practical use. Hereinafter, conversion of information into combinations of three of more kinds of numeric values is referred to as multileveling, and data subjected to multileveling are referred to as multilevel data. Additionally, a recording method that records information by multileveling in the aforementioned manner is referred to as a multilevel recording method.
In optical disks, because the distance between the adjacent pits (tracks) is short, it is difficult to focus an optical spot formed on a recording surface only on an intended part of the pit array (hereinafter referred to as a “target pit array”) or on an intended track (hereinafter referred to as a “target track”). Thus, the optical spot may be partially focused on a part of the pit array (hereinafter simply referred to as “adjacent pit array”) which part is adjacent to the target pit array. In this case, cross talk (leakage of unwanted signal) occurs between the target pit array and the adjacent pit array, and the detected signal would be a signal in which a signal from the adjacent pit array is superimposed on a signal from the target pit array. As a result, there is a possibility that the S/N ratio of a desired signal may be decreased. Particularly, in the multilevel recording method, because the influence of cross talk is significant compared to that in conventional binary recording methods, there is a possibility that reproducing timing may be shifted. Hence, an information recording medium has been proposed that is difficult to be influenced by cross talk in the multilevel recording method (for example, refer to Japanese Laid-open Patent Application No. 2003-85774).
However, though the information recording medium disclosed in Japanese Laid-open Patent Application No. 2003-85774 is effective in the cases where sectors are arranged in the CAV (Constant Angular Velocity) method, the ZCAV (Zoned CAV) method, and the ZCLV (Zoned Constant Linear Velocity) method, there is a possibility that this information recording medium may not be able to correspond to the CLV (Constant Linear Velocity) method, which can increase the storage capacity.